In recent years, control of pollutants in waste water as a measure for prevention of water pollution is being envisaged in many countries. It is necessary, for that purpose, to establish a method for quickly determining the concentration of pollutants in water as well as for measuring the quantity of water. For the purpose of measuring the concentration of pollutants in water, great importance has been assumed by the TOD (total oxygen demand) analyzer as an apparatus for monitoring the quality of water. With such apparatus nearly all organic materials are 100 percent detected. Also, measurement is very quick.
The TOD analyzer referred to above has a structure of the type, for instance, described in U.S. Pat. No. 3,560,156. When, however, attempts are made to measure the TOD of sea water with the use of a TOD analyzer having such a structure, serious problems arise.
That is to say, when the combustion supporting temperature is above 800.degree. C, fluctuations of the base line occur; and, at the same time, there is an increase in the so-called "blank value, " namely, the value measured when no combustible material is present in the test liquid.
In the measurement of, particularly, the TOD of a liquid of low concentration, such as the rather low range of tens of ppm, the values obtained vary widely and the reliability of the test is extremely low; it is virtually impossible to conduct an accurate measurement.
The above-mentioned phenomena are accounted for by the following facts:
While the melting point of sodium chloride is 800.4.degree. C and its boiling point is 1413.degree. C, it is of such a nature that, above its melting point, its vapor pressure is high and it sublimates. Therefore, sodium chloride contained in sea water is vaporized at a high temperature, and is oxidized by contact with the catalyst, thereby forming sodium hypochlorite and accordingly consuming oxygen in the feed gas which is intended for use in combustion of organic or other impurities.
Therefore, if the vapor pressure of the sodium chloride were to be lowered, the amount of sodium chloride oxidized by contact with the catalyst would decrease and, consequently, it should be possible to limit the fluctuation of the base line and the rise of the blank value. Accordingly, in the case of sea water, a method is used wherein the measurement is conducted at a lower temperature, for instance, 770.degree. C.
It is true that, after the combustion temperature has been lowered, there is reduced formation of sodium hypochlorite, and the aforesaid problems are resolved to a certain degree. This method is subject, however, to a major drawback in that with the lowering of the combustion temperature, the rate of oxidization of organic materials decreases, and errors in the values obtained from the measurement become greater. From this reason, the combustion temperature is, in the method, set at 770.degree. C, such being regarded as the temperature at which sodium hypochlorite is not likely to be formed as mentioned above, and yet, the temperature is still high enough that organic materials are easily oxidized.
As a consequence, in the foregoing method, the TOD of sea water is measured at 770.degree. C and that of ordinary waste water is measured at 900.degree. C. Accordingly, the method has a drawback in that, on account of the existence of this difference in temperature, it is impossible alternately to measure the TOD of sea water and that of ordinary waste water, or, even if it were at all possible, it would take a long time to stabilize the apparatus. The conventional method of measuring the TOD of sea water makes it a prerequisite, as heretofore mentioned, to prevent as far as practicable, the formation of sodium hypochlorite. As a natural consequence, the process of measurement becomes complex. Further, those components contained in sea water which are less combustible remain wholly or partially unburned. Hence errors arise in the values obtained from the measurements.
The present invention eliminates the shortcomings of the prior art, and one of its main objects is to conduct a measurement of the TOD of sea water at the same combustion temperature as with ordinary waste water, without experienceing undesirable fluctuations in the base line and in the blank values.